The export coal terminals in the US have a very high throughput and utilize very large conveyor belts for loading vessels. These operations are typically operated 24/7 and sampling must be continuous as well since the product is highly sensitive to changing coal qualities. In the event the primary sampling system becomes inoperable, the traditional acceptable method of sampling has been to temporarily sample manually.
Manual sampling basically requires a man with a shovel in hand being given the task of collecting shovels of coal from a moving conveyor. This means of sampling has always been a dangerous task but has become especially so since the larger operations are moving to even larger conveyors and higher feed rates.
Due to the fact that the chemical characteristics of commercial interest are often distributed unequally in the different sized particles in a consignment—and that solid bulk cargos segregate by size when handled—the capturing of the particle size distribution of the consignment in the sample increments is critical to collecting an unbiased sample.
The fundamental rule of sampling a bulk cargo is that each particle of equal mass in a consignment must have an equal chance of being selected in the sample. However, this requirement is not met by all ASTM sampling methods.
According to ASTM Standard D 2234, there are four Conditions (or Types) of Sampling:                Condition A—Stopped Belt Sampling        Condition B—Full Stream Sampling        Condition C—Part Stream Sampling        Condition D—Stationary Sampling (Stockpiles, barges, railcars, etc.)        
Conditions A and B are Probability Sampling—because each particle in the consignment has a non-zero chance of being selected for the sample. When executed properly, these can overcome particle size segregation, or moisture migration, in the consignment.
Conditions C and D are considered to be Non-Probability Sampling—because some particles in the consignment have zero chance of being selected. Even when executed properly, these cannot always overcome particle size segregation or moisture migration. For this reason, Non-Probability Sampling is often called “Judgment sampling” (as in human judgment).
What is needed in the industry is a mechanical sampling system that can be used safely and effectively while the permanent sampling system is unavailable for use temporarily.